This invention relates to a heat ray reflecting material capable of efficiently reflecting heat ray radiated from an exothermic body, and a heating apparatus using the same.
Manufacturing process for semiconductor wafers and device fabrication process using the semiconductor wafers include processes in which the semiconductor wafers are heated to several-hundred to thousand-and-several-hundred degree centigrade, and for which a various types of annealing furnaces such as those based on resistance heating system (heater heating system), lamp heating system and so forth are used depending on purposes.
For example, in order to manufacture a silicon single crystal wafer, which is a representative semiconductor wafer, a silicon single crystal ingot is pulled using a single crystal pulling apparatus. In the pulling of the single crystal ingot, a quartz crucible in which polysilicon is charged is surrounded by a heater, and the crucible is heated to a temperature as high as 1,420xc2x0 C. so as to fuse the polysilicon as a source material. After the silicon single crystal thus produced is processed to obtain the wafers, removal of damage, diffusion of impurities, vapor phase growth of semiconductor films and so forth are also carried out under heated atmosphere, where a variety of heating apparatuses are used therefor. Also in the field of compound semiconductor, heating apparatuses are used for vapor phase growth or liquid phase growth of semiconductor films, and other annealing processes.
In order to raise the heating efficiency, the annealing apparatuses used for the above-described annealing are generally configured so that heat insulating materials are disposed around the exothermic body such as the aforementioned heater or lamp so as to prevent heat from dissipating to the externals. In more simplified heating apparatuses, the heat insulating material may sometimes omitted for size reduction.
Disposition of the heat insulating material, however, not only increases the size of the heating apparatus but also needs a longer time additionally for heating of the heat insulating material because of a large heat capacity thereof, and still also for cooling after completion of the annealing. Disposition of a forced cooling apparatus based on water cooling, air cooling or the like further increases the size of the apparatus. Moreover, heat absorbed by the heat insulating material is of course less contributable to heating of the works, and this worsens the energy efficiency. It is a matter of course that the apparatus using no heat insulating material further suffers from more waste dissipation of the energy.
A subject of this invention is therefore to provide a heat ray reflecting material capable of reflecting heat ray emitted from an exothermic body in an extremely efficient manner, and a heating apparatus capable of efficiently raising or lowering the temperature by concentrating heat ray emitted from an exothermic body towards the works by using the heat ray reflecting material.
To solve the aforementioned subject, a heat ray reflecting material of the invention is such as being capable of reflecting heat ray in a specific wavelength band, being a stack of a plurality of element reflecting layers comprising materials having transparent properties to the heat ray, wherein, in the element reflecting layers, two adjacent layers are composed of a combination of materials differed from each other in refractive indices to the heat ray, while keeping difference between the refractive indices of 1.1 or larger.
The heat ray reflecting material of the invention is configured based on a combination of the element reflecting layers respectively having transparent properties to the heat ray, differed from each other in refractive indices to the heat ray, and keeping difference between the refractive indices of 1.1 or larger. By composing the heat ray reflecting material based on the combination of the element reflecting layers ensuring a large difference in the refractive indices therebetween, the heat ray can be reflected at an extremely high reflectivity. Because a high reflectivity can be achieved only by a limited range of increase in the number of stacking of the element reflecting layers, the heat ray reflecting material can be manufactured at low costs. Difference in the refractive indices less than 1.1 inevitably lowers the reflectivity, and increase in the number of cycles of the stacking intended for an improved reflectivity raises the costs. The difference in the refractive indices between the element reflecting layers to be combined is preferably kept at 1.2 or more, more preferably 1.5 or more, and still more preferably 2.0 or more.
While xe2x80x9chaving transparent propertiesxe2x80x9d herein can be defined as a status that an object has a property of allowing electromagnetic wave such as light to pass therethrough, transparent property in this invention is preferably such as ensuring a transmissivity of the heat ray to be reflected of as large as 80% or more for the thickness to be adopted. The transmissivity less than 80% may increase the absorption ratio of the heat ray, and may prevent the heat ray reflecting material of the invention from fully exhibiting the effect of reflecting the heat ray. The transmissivity is preferably 90% or more, and more preferably 100%. A transmissivity of 100% herein means a transmissivity which can be considered as almost 100% within a range of measurement limit (e.g., within xc2x11% error) in general methods of measuring transmissivity.
Next, a heating apparatus according to a first aspect of this invention comprises a container having a work housing space formed therein; a heat source for heating a work in the work housing space; and a heat ray reflecting member having a heat reflecting surface thereof composed of the heat ray reflecting material of this invention, so as to allow the heat ray generated in the work housing space to reflect on the heat reflecting surface to thereby change the direction thereof towards the work.
A heating apparatus according to a second aspect of this invention comprises at least an annealing chamber for carrying out annealing; an exothermic body disposed outside the annealing chamber; and a heat ray reflecting member surrounding the exothermic body and the annealing chamber and having a heat reflecting surface thereof composed of the heat ray reflecting material of this invention.
By applying the heat ray reflecting member composed of the heat ray reflecting material of this invention to a heating apparatus, and by using the member as a substitute, for example, for a part of or entire portion of the heat insulating material, delay in the heating or cooling speed of the annealing apparatus ascribable to heat capacity of the heat insulating material can be improved, and this enables more rapid heating and cooling as compared with those for the conventional apparatus. It is also possible to expand a length of uniform heating as compared with that for the conventional annealing apparatus. It is still also possible to achieve down-sizing and energy-saving effects of the apparatus because energy of the heat ray from the exothermic body can efficiently be concentrated to the work.
The specific wavelength band of the heat ray selected from a range of 1 to 10 xcexcm can cover wavelength ranges of heat ray necessary for heat processing in various applications, and can promise the effect of this invention. Although applicable fields of the heating apparatus of this invention are not specifically limited, one possible example is like the followings. In order to manufacture a silicon single crystal wafer which is a representative semiconductor wafer, first a silicon single crystal ingot is pulled using a single crystal pulling apparatus. In the pulling of the single crystal ingot, a quartz crucible in which polysilicon is charged is surrounded by a heater, and the crucible is heated to a temperature as high as 1,420xc2x0 C. or above. The invention is applicable to this heating apparatus. In this case, the wavelength band of the heat ray to be reflected falls within a range from 1 to 5 xcexcm, and more preferably 1 to 3 xcexcm, which successfully covers an essential portion of spectra of the heat ray emitted from a melt of semiconductor source material or from the heater for keeping a molten state of the semiconductor source material. This makes it possible to efficiently reflect and control the radiated infrared ray.
After the silicon single crystal thus produced is processed to obtain the wafers, removal of damage, diffusion of impurities, vapor phase growth of semiconductor films and so forth are carried out also under heated atmosphere as high as 400 to 1,400xc2x0 C., for example, where the invention is applicable also to various heating apparatuses for carrying out these processes. In these cases, the wavelength band of the heat ray to be reflected falls within a range from 1 to 30 xcexcm, and more preferably 1 to 10 xcexcm. This makes it possible to efficiently reflect and control the heat ray from a heat source such as the heater or the like, or from the work per se heated therewith.
On the other hand, heating apparatuses are used also in the field of compound semiconductor for vapor phase growth or liquid phase growth of semiconductor films, or other annealing processes (temperature range: 400 to 1,400xc2x0 C. or around), where the invention is again applicable thereto. The wavelength band of the heat ray to be reflected falls within a range from 1 to 30 xcexcm, and more preferably 1 to 10 xcexcm.
Moreover, heat processing is applied not only for the aforementioned semiconductor materials, but is widely applied to various materials or to a vast variety of processes. For example, various heating apparatuses are used in the manufacture of metal materials or metal members, for melting of the materials, sintering, hot working and other annealing processes (temperature range: 400 to 1,800xc2x0 C. or around, wavelength band of heat ray to be reflected: 0.3 to 30 xcexcm). Also in the manufacture of inorganic materials such as ceramics or glasses, heating apparatuses are used for sintering, processing or other annealing processes (temperature range: 700 to 1,800xc2x0 C. or around, wavelength band of heat ray to be reflected: 0.3 to 20 xcexcm). Besides these, drying furnace or the like used for various applications can be said as a kind of heating apparatus. Further besides those of industrial use, heating cooking instruments (e.g., oven) for business use or home use can be exemplified. These are used at a relatively low temperature range, for example, 200 to 500xc2x0 C. or around (wavelength band of heat ray to be reflected: 3 to 40 xcexcm).
The stack composing the heat ray reflecting member can be configured so as to include first and second element reflecting layers differed in refractive indices and being adjacent to each other, and so that stack cycle units, each of which comprising a first and a second element reflecting layers, are formed in two or more cycles on the surface of a base member. The periodic changes in the refractive index of the stack in the thickness-wise direction thereof is successful in further raising the heat ray reflectivity. In this case, a larger difference in the refractive indices of the plurality of materials composing the stack cycle units can result in a larger reflectivity. For example, the simplest constitution of the stack cycle unit relates to a two-layered structure comprising the first element reflecting layer and the second element reflecting layer differed in the refractive indices to the heat ray from each other. In this case, a larger difference between the refractive indices of both layers can reduce the number of the stack cycle units necessary for keeping the reflectivity of heat ray at a sufficiently high level. The number of layers composing the stack cycle unit may be three or more.
The thickness of the stack cycle unit can be set smaller than the central wavelength of the heat ray to be reflected. This setting is successful in the formation of an optical stop band structure (or one-dimensional photonic band gap structure) against the heat ray of a specific wavelength band in the thickness-wise direction of the heat reflecting layer depending on the distribution of refractive indices in the stack cycle units, and this makes it possible to almost completely reflect the heat ray of such specific wavelength band to thereby fully extract the aforementioned effects of the invention. The thickness of the individual layers and the number of cycles can be determined by calculations or experiments based on a range of the wavelength band to be reflected. Adoption of the combination of the materials differed in the refractive indices by 1.1 or more as in the invention is successful in readily realizing the stack cycle structure having a heat ray reflectivity close to complete reflection with a relatively small number of formation cycles of the stack cycle unit, or more specifically 5 cycles or less. In particular, adoption of a combination ensuring a difference in the refractive indices of 1.5 or more is successful in realizing a large heat ray reflectivity as described in the above only with the number of formation of cycles of 4 cycles, 3 cycles or even as small as 2 cycles.
The range of the wavelength band to be reflected depends on the temperature of the heat source. That is, of the radiated energies emitted from a unit area of object surface within a unit time under a predetermined temperature, the maximum limit energy is shown by monochromatic emissive power obtained from a perfect black body. This can be expressed by the equation below (Planck""s Law):
Ebxcex=Axcexxe2x88x925(eB/xcexTxe2x88x921)xe2x88x921[W/(xcexcm)2]
where, Ebxcex: monochromatic emissive power of black body [W/(xcexcm)2], xcex: wavelength [xcexcm], T: absolute temperature of object surface [K], A: 3.74041xc3x9710xe2x88x9216[Wxc2x7m2], and B: 1.4388xc3x9710xe2x88x922[mxc2x7K]. FIG. 8 is a graph showing relations between monochromatic emissive power (Ebxcex) and wavelength obtained when absolute temperature T of the object surface was varied. It is seen that the monochromatic emissive power peak lowers and shifts towards the longer wavelength side as T decreases.
Materials of the element reflecting layer composing the stack are preferably selected from combinations of the materials which are stable against high temperatures and capable of ensuring a necessary-and-sufficient level of difference in the refractive indices therebetween for infrared reflection. The stack can be configured so as to include a layer which comprises a semiconductor or an insulator having a refractive index of 3 or above, as the first element reflecting layer which serves as a high-refractive-index layer. Use of the semiconductor or an insulator having a refractive index of 3 or above as the first element reflecting layer facilitates to ensure a large difference in the refractive index from that of the second element reflecting layer to be combined therewith. Table 1 summarizes refractive indices of materials for the element reflecting layers applicable to the invention. Substances having refractive indices of 3 or above can be exemplified by Si, Ge and 6h-SiC, and also by compound semiconductors such as Sb2S3, BP, AlP, AlAs, AlSb, GaP and ZnTe. As for semiconductors and insulators, it is preferable to use those having band gap energies sufficiently larger (e.g., 2 eV or above) than photon energies of the heat ray, because those of the direct transition type having band gap energies close to the photon energies of the heat ray to be reflected tend to absorb the heat ray. On the other hand, those of indirect transition type (e.g., Si, Ge) can suppress the heat ray absorption at a low level and applicable to the invention in a desirable manner even if the band gap energies thereof are smaller than the above-described value. Among others, Si is advantageously used for the invention because it has a relatively low price, can readily be made in a form of a thin film, and has a refractive index of as large as 3.5. Using a Si layer as the first element reflecting layer can therefore realize a stacked structure having a high reflectivity at low costs.
Next, low-refractive-index materials for composing the second element reflecting layer can be exemplified by SiO2, BN, AlN, Al2O3, Si3N4 and CN. In this case, it is necessary to select the materials for the second element reflecting layer so as to ensure a difference in the refractive index of 1.1 or more depending on the types of the materials already selected for the first element reflecting layer. Table 1 below summarizes refractive indices of these materials. Among others, adoption of SiO2 layer, BN layer or Si3N4 layer is particularly advantageous in ensuring a large difference in the refractive indices. The SiO2 layer has a refractive index of as small as 1.5, and can ensure a large difference from that of the first element reflecting layer typically composed of a Si layer. It is also advantageous in that it can readily be formed typically by oxidation of the Si layer. On the other hand, the BN layer has a refractive index ranging from 1.65 to 2.1 depending on the crystal structure or orientation. The Si3N4 layer has a refractive index ranging from 1.6 to 2.1 depending on quality of the film. Although these layers have slightly larger refractive indices as compared with that of SiO2, they can still ensure a large difference of refractive index as much as 1.4 to 1.85 from that of Si. Considering the temperature range (400 to 1,400xc2x0 C.) generally used for manufacturing of silicon wafers, it is effective, in view of efficiently reflecting the radiated heat, to compose the heat ray reflecting layer so as to essentially include a Si layer and additionally include at least either of a SiO2 layer or a BN layer, and more specifically to compose it so that the Si layer and SiO2 layer and/or the BN layer are included as the element reflecting layer. The BN layer is desirable when applied to ultra-high-temperature use since it has a melting point considerably higher than that of SiO2. BN is also advantageous in that it can emit only N2 as an outgas if decomposed at high temperatures, where boron remains in the surface in a semi-metallic state, and is thus not affective to electric characteristics of the semiconductor wafers such as Si wafer and the like. Table 2 shows exemplary combination of preferable materials by temperature zones.
Next paragraphs will describe results of calculative examination on conditions which can ensure an almost complete reflection of infrared region by forming an one-dimensional photonic band gap structure using Si and SiO2. Si has a refractive index of approximately 3.5, and a thin film thereof is transparent to light in the infrared region having a wavelength of approximately 1.1 to 10 xcexcm. SiO2 has a refractive index of approximately 1.5, and a thin film thereof is transparent to light in the infrared region having a wavelength of approximately 0.2 to 8 xcexcm (visible to infrared regions). FIG. 1 is a sectional view of a heat ray reflecting layer having formed on Si substrate 100, four cycles of the stack cycle units, each of which comprising two layers of a Si layer xe2x80x9cAxe2x80x9d of 100 nm thick and a SiO2 layer xe2x80x9cBxe2x80x9d of 233 nm thick. This structure can achieve a reflectivity of infrared radiation in the 1 to 2 xcexcm band of nearly 100% as shown in FIG. 2, and can inhibit transmission of the infrared radiation. It is also allowable to compose the base member with other materials (e.g., quartz (SiO2)), another Si layer is formed thereon, and stack cycle unit similarly comprising two layers of a Si layer xe2x80x9cAxe2x80x9d and SiO2 layer xe2x80x9cBxe2x80x9d can be formed further thereon.
For example, a heat source of 1,600xc2x0 C. has a maximum intensity in 1 to 2 xcexcm band, where coverage of this band together with 2 to 3 xcexcm band (corresponding to a peak wavelength region of heat ray spectrum obtained from a heat source of around 1,000 to 1,200xc2x0 C.) is accomplished by adding another periodic combination differed in the wavelength region to be reflected. That is, an allowable configuration may be such as shown in FIG. 3, where the aforementioned combinations of 100 nm (Si)/233 nm (SiO2) are added with another thickened combinations of 157 nm (Si)/366 nm (SiO2) (Axe2x80x2/Bxe2x80x2 in FIG. 3).
In contrast to that the above-described 4-cycle structure of 100 nm (Si)/233 nm (SiO2) ensures nearly 100% reflectivity for infrared radiation in 1 to 2 xcexcm band, the 4-cycle structure of 157 nm (Si)/366 nm (SiO2) ensures nearly 100% reflectivity for infrared radiation in 2 to 3 xcexcm band, as shown in FIG. 4. Therefore the structure shown in FIG. 3 in which these structures are stacked can provide a material which can ensure nearly 100% reflectivity over 1 to 3 xcexcm band.
Similarly, 3 to 4.5 xcexcm band can be covered by properly selecting a further thickened combination of Si layer and SiO2 layer and by forming the 4-cycle structure. Combination of layer only capable of ensuring a difference between the refractive indices smaller than that between Si and SiO2 may need a larger number of cycles, so that a larger difference between two layers to be selected is more advantageous. According to the above-described combination, a total thickness of 1.3 xcexcm ensures almost complete reflection of 1 to 2 xcexcm band, and a total thickness of 3.4 xcexcm ensures that of 1 to 3 xcexcm band.
On the other hand, FIG. 5 shows a calculated result of the reflectivity of the heat ray reflecting layer having a 4-cycle structure of 94 nm (SiC)/182 nm (BN), based on selection of 6h-SiC (refractive index 3.2) and h-BN (refractive index 1.65) capable of ensuring a large difference in the refractive indices therebetween similarly to the case of Si and SiO2. It is known in this case that reflectivity of nearly 100% is achieved for light (heat ray) in 1 to 1.5 xcexcm band.